A tractor is an example of work vehicle. Some tractors include a suspension mechanism at front wheels as disclosed in Patent Document 1, for example. In general, a suspension mechanism provides an operator with a riding comfort by absorbing unevenness of the ground surface in association with upward/downward movement thereof according to the ground surface unevenness.
A work site where the work vehicle travels has more unevenness than a paved road and the work vehicle is often equipped with an implement. Therefore, a suspension stroke position of the suspension mechanism sometimes deviates from a preset target range (when the suspension stroke position of the suspension mechanism is located within the target range, the vehicle height is maintained within a predetermined height range) either to the vehicle body elevating side or vehicle body lowering side, whereby the vehicle height may deviate (change extraordinarily) from a predetermined height range.
In this case, it is desired for the work vehicle to restrict such an extraordinary change in vehicle height, thereby to constantly maintain the vehicle height within the predetermined height range. Hence, the art has proposed to maintain the vehicle height within the predetermined height range as much as possible, by means of a suspension reference position varying mechanism configured to move the suspension stroke position of the suspension mechanism toward the target range when the suspension stroke position of the suspension mechanism deviates from the target range either to the vehicle elevating side or vehicle lowering side as described above.
Patent Document 1: United States Patent No. 6145859
With the suspension system for a work vehicle described above, when the suspension mechanism is provided with the above-described suspension reference position varying mechanism for varying the reference position of the suspension stroke of this suspension mechanism, there occurs a problem as described next. That is, when the suspension mechanism is operative and if the suspension reference position varying mechanism is activated in association with each and every occurrence of deviation of the suspension stroke position of the suspension mechanism from the target range to the vehicle elevating side or the vehicle lowering side, then, the operation frequency of the suspension reference position varying mechanism will be very high, which is disadvantageous from the viewpoint of the durability of this suspension reference position varying mechanism.